Hi, and welcome. Congrats on your upcoming purchase!
First, I would definitely check to be sure that it doesnt have existing mods on there already. Based on my experience, alot of the time...dealers might not even know. Its hard to comeby a evo 8/9 that doesnt have mods already. Its possible for a evo 9 to come stock though, since its more recent.
I would definitely get the basic bolt ons - drop-in filter, turbo back exhaust, downpipe or test pipe or highflow cat(depending on where you live and your emission regulations), manual boost controller, fuel pump and a tune. You will definitely notice a difference once you have those basic things. You should get use to how your car runs with these additional mods then move on to upgrading more. Also, suspension (coilovers or springs) is great as well for a first mod.
You can also add all the other possible boltons like upgrading your front mount intercooler, changing all your piping (upper and lower), mini battery, cams and headstuds. Cams make a big difference in power for the 8/9s. If all else fails and you crave more power, get a new turbo kit. Definitely do your reserach first before chosing the right one. A lot of people have different goals for their cars wether it be a time attack car, track car or show car and how you mod your car for whichever goal you want makes a difference.

Good luck.